Optical fibers are conventionally made from glass rods or "preforms" which have a central core of glass enveloped by a cladding of glass whose refractive index is lower than that of the glass core. The fibers are produced by heating the glass preform to softening temperature in a furnace and drawing fiber from the softened preform. The fiber is rapidly cooled sufficiently to enable a protective coating of resin material to be applied to the surface of the drawn fiber. The cooling is carried out by drawing the fiber through a draw tower with a heat exchanger wherein it comes into contact with a gas coolant, which is generally helium, although it can be other gases, such as nitrogen, carbon dioxide or hydrogen. The gas coolant is continually passed through the heat exchanger, generally in a crossflow or counterflow direction, relative to the direction of movement of the glass fiber through the heat exchanger. The gaseous coolant transfers heat from the glass fiber to a cooling medium, usually water, which passes through a compartment which surrounds the cooling chamber. Helium is the preferred coolant gas because it has good heat transport properties and is safe to use. Helium is costly relative to other gases, however, so it is desirable to recycle it for reuse in the heatexchanger.
One characteristic of currently practiced glass fiber cooling processes is that the openings through which the glass fiber enters and exits the cooling chamber are not gas tight. Because of this, air usually infiltrates the system and dilutes the gaseous coolant, which eventually makes it necessary to discard the coolant or purify it for reuse. Air infiltration can be reduced significantly by ensuring that there is a positive differential between the pressure in the cooling chamber and that in the surrounding environment. In other words, the pressure in the cooling chamber is maintained above that of the surrounding atmosphere. This has the disadvantage that valuable helium will be lost to the environment through the fiber inlet opening and/or the fiber outlet opening of the cooling chamber.
Efforts have been made to minimize the amount of helium efflux and air influx through the fiber inlet and outlet openings. U.S. Pat. Nos. 5,377,491 and 5,452,583 disclose a process and system for cooling optical fiber by cooling the fiber with a coolant gas, such as helium, in heat exchangers. This patent teaches controlling the flow of cooling gas into and out of the heat exchangers to limit air infiltration into the heat exchangers. The disclosed process involves the introduction of coolant gas into the heat exchangers at a pressure of about 0 to about 150 psig. Unfortunately, operating the heat exchangers at atmospheric or superatmospheric pressure results in a significant loss of coolant gas from the system, stated to be less than about 50%.
More economical processes for producing optical glass fiber are constantly sought. The present invention provides a process which reduces the cost of producing hot glass fibers by minimizing the amount of gaseous coolant lost to the environment during the fiber cooling process.